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<div id="dev-warning" class="admonition note">
  <p class="first admonition-title">Notice</p>
  <p class="last">This document is for a development version of Ceph.</p>
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  <div class="section" id="id1">
<h1>去重<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h1>
<div class="section" id="id2">
<h2>介绍<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h2>
<p>Applying data deduplication on an existing software stack is not easy
due to additional metadata management and original data processing
procedure.</p>
<p>In a typical deduplication system, the input source as a data
object is split into multiple chunks by a chunking algorithm.
The deduplication system then compares each chunk with
the existing data chunks, stored in the storage previously.
To this end, a fingerprint index that stores the hash value
of each chunk is employed by the deduplication system
in order to easily find the existing chunks by comparing
hash value rather than searching all contents that reside in
the underlying storage.</p>
<p>There are many challenges in order to implement deduplication on top
of Ceph. Among them, two issues are essential for deduplication.
First is managing scalability of fingerprint index; Second is
it is complex to ensure compatibility between newly applied
deduplication metadata and existing metadata.</p>
</div>
<div class="section" id="key-idea">
<h2>Key Idea<a class="headerlink" href="#key-idea" title="Permalink to this headline">¶</a></h2>
<p>1. Content hashing (Double hashing): Each client can find an object data
for an object ID using CRUSH. With CRUSH, a client knows object’s location
in Base tier.
By hashing object’s content at Base tier, a new OID (chunk ID) is generated.
Chunk tier stores in the new OID that has a partial content of original object.</p>
<blockquote>
<div><p>Client 1 -&gt; OID=1 -&gt; HASH(1’s content)=K -&gt; OID=K -&gt;
CRUSH(K) -&gt; chunk’s location</p>
</div></blockquote>
<p>2. Self-contained object: The external metadata design
makes difficult for integration with storage feature support
since existing storage features cannot recognize the
additional external data structures. If we can design data
deduplication system without any external component, the
original storage features can be reused.</p>
<p>More details in <a class="reference external" href="https://ieeexplore.ieee.org/document/8416369">https://ieeexplore.ieee.org/document/8416369</a></p>
</div>
<div class="section" id="design">
<h2>Design<a class="headerlink" href="#design" title="Permalink to this headline">¶</a></h2>
<p>Pool-based object management:
We define two pools.
The metadata pool stores metadata objects and the chunk pool stores
chunk objects. Since these two pools are divided based on
the purpose and usage, each pool can be managed more
efficiently according to its different characteristics. Base
pool and the chunk pool can separately select a redundancy
scheme between replication and erasure coding depending on
its usage and each pool can be placed in a different storage
location depending on the required performance.</p>
<p>Regarding how to use, please see <code class="docutils literal notranslate"><span class="pre">osd_internals/manifest.rst</span></code></p>
</div>
<div class="section" id="usage-patterns">
<h2>Usage Patterns<a class="headerlink" href="#usage-patterns" title="Permalink to this headline">¶</a></h2>
<p>The different Ceph interface layers present potentially different oportunities
and costs for deduplication and tiering in general.</p>
<div class="section" id="radosgw">
<h3>RadosGW<a class="headerlink" href="#radosgw" title="Permalink to this headline">¶</a></h3>
<p>S3 big data workloads seem like a good opportunity for deduplication.  These
objects tend to be write once, read mostly objects which don’t see partial
overwrites.  As such, it makes sense to fingerprint and dedup up front.</p>
<p>Unlike cephfs and rbd, radosgw has a system for storing
explicit metadata in the head object of a logical s3 object for
locating the remaining pieces.  As such, radosgw could use the
refcounting machinery (<code class="docutils literal notranslate"><span class="pre">osd_internals/refcount.rst</span></code>) directly without
needing direct support from rados for manifests.</p>
</div>
<div class="section" id="rbd-cephfs">
<h3>RBD/Cephfs<a class="headerlink" href="#rbd-cephfs" title="Permalink to this headline">¶</a></h3>
<p>RBD and CephFS both use deterministic naming schemes to partition
block devices/file data over rados objects.  As such, the redirection
metadata would need to be included as part of rados, presumably
transparently.</p>
<p>Moreover, unlike radosgw, rbd/cephfs rados objects can see overwrites.
For those objects, we don’t really want to perform dedup, and we don’t
want to pay a write latency penalty in the hot path to do so anyway.
As such, performing tiering and dedup on cold objects in the background
is likely to be preferred.</p>
<p>One important wrinkle, however, is that both rbd and cephfs workloads
often feature usage of snapshots.  This means that the rados manifest
support needs robust support for snapshots.</p>
</div>
</div>
<div class="section" id="rados-machinery">
<h2>RADOS Machinery<a class="headerlink" href="#rados-machinery" title="Permalink to this headline">¶</a></h2>
<p>For more information on rados redirect/chunk/dedup support, see <code class="docutils literal notranslate"><span class="pre">osd_internals/manifest.rst</span></code>.
For more information on rados refcount support, see <code class="docutils literal notranslate"><span class="pre">osd_internals/refcount.rst</span></code>.</p>
</div>
<div class="section" id="status-and-future-work">
<h2>Status and Future Work<a class="headerlink" href="#status-and-future-work" title="Permalink to this headline">¶</a></h2>
<p>At the moment, there exists some preliminary support for manifest
objects within the OSD as well as a dedup tool.</p>
<p>RadosGW data warehouse workloads probably represent the largest
opportunity for this feature, so the first priority is probably to add
direct support for fingerprinting and redirects into the refcount pool
to radosgw.</p>
<p>Aside from radosgw, completing work on manifest object support in the
OSD particularly as it relates to snapshots would be the next step for
rbd and cephfs workloads.</p>
</div>
</div>



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